Until the nineteen-seventies, solar heating received only scattered, spasmodic attention because it was not price competitive with the use of fossil fuels. Renewed interest in solar energy has developed during the past few years as a result of increasing costs of energy from fossil fuels, the problems of depletion of resources, and the degradation of the environment. Although solar energy may be considered as a new and unconventional resource, it has been used for many centuries for drying agricultural products, heating water, etc.
Prior art solar heating systems are generally accomplished by placing large collector plates on the roof and side structure of buildings. Liquids, such as water, are piped through the collector, heated to a higher temperature, and subsequently circulated through a building and used as a space and tap-water heating medium. Such a system may be useful during the sunlight hours but loses its effectiveness after sundown. In a more elaborate application of the same principle, it has been suggested to place large tanks of water, rocks, stones, etc., in the ground and have heated water from the solar collector fed into the tanks, where the heat energy is stored and utilized for space and hot water heating. But, during prolonged cloudy or rainy weather, such a system loses its effectiveness because of its incapacity to store more than a few days' supply of heat energy. Furthermore, during the winter period when the heat is most desirable, the efficiency of the solar radiation is much less than that of summer months, and heat loss from the collector to the surroundings is much greater. In short, currently known solar-heating systems must be installed in addition to, not instead of, conventional heating systems.
The earth, as a thermal storage reservoir, has several interesting attributes. First, heat capacities are extremely large because of the large available mass. For example, in a volume of one acre of land, 15 feet deep, at a temperature difference of 50.degree. F. the energy flux is estimated to be 1.49 .times. 10.sup.9 BTU. The area of one acre is much larger than an average house. Specifically, it is over 20 to 40 times larger than the average house of sizes from 2,000 ft.sup.2 to 1,000 ft.sup.2. Assuming a winter heating requirement of 60 .times. 10.sup.6 BTU for a small house, such a volume of earth would be able to store sufficient heat to supply approximately 25 homes.
Another attribute of the earth is its extremely low thermal conductivity. Since the storage system may be unbounded in the downward and sideward directions as opposed to the confined, insulated and water-proofed constructions of the conventional designs, the low thermal conductivity of the earth restricts losses in those directions. It has been approximated that the total energy in the solar flux over one acre of area, for one summer season, is 1.2 .times. 10.sup.10 BTU; these are approximately the heating requirements for 200 houses. It can therefore be seen that the theoretically available energy from the sun is extremely high.